By the time food and digestive juices reach your large intestine, most digestion and nutrient absorption has already taken place. The large intestine’s major jobs are to absorb excess water and to prepare feces -- or undigested waste material -- for removal via the rectum and anus.

The Basics

Your large intestine, which directly follows your small intestine, is about 5 feet long. Like the small intestine, it’s split into three sections -- the cecum, the colon and the rectum. The colon, which is the largest section, is responsible for most of the work of the large intestine.

The Cecum

Your small intestine allows about 3 cups of unabsorbed residue to travel into the cecum each day. The residue enters the large intestine through the ileocecal sphincter, which allows material to pass into the cecum but prevents the backflow of fecal matter into the small intestine. At this point, the residue contains undigested food -- or fiber, water, and some vitamins, minerals and salts.

The Colon

The colon -- the middle part of the large intestine -- is separated into four sections -- the ascending colon, the transverse colon, the descending colon and the sigmoid colon. Once the undigested residue enters the colon, it mixes with mucus and bacteria that live in the large intestine and begins the formation of fecal matter. As the fecal matter moves through the colon, the colon absorbs most of the water and some of the remaining vitamins and minerals. The bacteria that live in your colon use the material to produce some vitamins, like B vitamins, biotin and vitamin K. The bacteria also turn some of the unabsorbed carbohydrates into methane gas, carbon dioxide and hydrogen. The remaining matter is propelled toward the rectum, the final portion of the large intestine.

The Rectum

When fecal matter reaches the rectum, the final 8-inch section of the large intestine, it triggers the defecation reflex -- the signal that lets you know you have to go to the bathroom. At this point, you either eliminate the fecal matter through the anus or it remains stored in the rectum until you do.

About the Author

Lindsay Boyers has a Bachelor of Science in nutrition from Framingham State College and a certificate in holistic nutrition from the American College of Healthcare Sciences. She is also a licensed aesthetician with advanced training in skincare and makeup. She plans to continue on with her education, complete a master's degree program in nutrition and, ultimately, become a registered dietitian.

bibliography-iconicon for annotation tool Cite this Article

Choose Citation Style

Boyers, Lindsay. "What Happens to the Undigested Food Once It Gets Into the Large Intestine?" Healthy Eating | SF Gate, http://healthyeating.sfgate.com/happens-undigested-food-once-gets-large-intestine-11991.html. 02 December 2018.

Boyers, Lindsay. (2018, December 02). What Happens to the Undigested Food Once It Gets Into the Large Intestine? Healthy Eating | SF Gate. Retrieved from http://healthyeating.sfgate.com/happens-undigested-food-once-gets-large-intestine-11991.html

Boyers, Lindsay. "What Happens to the Undigested Food Once It Gets Into the Large Intestine?" last modified December 02, 2018. http://healthyeating.sfgate.com/happens-undigested-food-once-gets-large-intestine-11991.html

Note: Depending on which text editor you're pasting into, you might have to add the italics to the site name.